1. Field of the Invention
The present invention relates to a misfire detecting device for a water jet propulsion watercraft that has an engine and a jet pump operated by the engine to propel the watercraft.
2. Description of the Related Art
Some kinds of water jet propulsion watercrafts are constructed to prevent mechanical troubles from occurring when an engine rotates in an excessively high speed range by controlling the engine to misfire at a predetermined misfire rate if an engine speed reaches a predetermined engine speed (for example, see JP-B-3321007). A water jet propulsion watercraft disclosed in JP-B-3321007 retards ignition timing of the engine when the engine speed reaches an engine speed set at a level immediately before a predetermined high speed. If the engine speed exceeds the predetermined high speed even though such a control is implemented, the water jet propulsion watercraft raises the misfire rate of the engine to be higher than the misfire rate given immediately before the engine speed reaches the predetermined high speed and also advances the ignition timing.
Thus, the engine speed decreases when the engine speed is about to reach the predetermined high speed so that the engine speed is restrained to be below the predetermined high speed. If, however, the engine speed reaches the predetermined high speed notwithstanding the restraint, the misfire rate is raised to effectively decrease the engine speed. Thereby, the engine speed is more surely prevented from largely exceeding the predetermined high speed. Accordingly, engine troubles caused by excessive operations can be prevented.
However, engines often have not only the intentional misfire caused under such a control but also unintentional misfires caused due to damage or deterioration of ignition plugs. Particularly, engines for water jet watercrafts have a lot of potential for unintentional misfires because water can more easily enter the inside of watercraft engines than engines for automobiles or the like. If such an unintentional misfire occurs, unburnt gases remain and deteriorate the properties of exhaust gases. After-fire thus can occur in an exhaust conduit. In addition, if a catalyst is disposed inside of the exhaust conduit, the catalyst can be deteriorated by the after-fire.
Hence, it is desirable to detect the occurrence of the misfire and warn an operator of the misfire and further to implement an engine control whereby both the deterioration of the properties of exhaust gases and the deterioration of the catalyst can be minimized. However, the conventional water jet propulsion watercraft described above can have not only the abnormal misfire but also the intentional misfire generated by the controller. If, therefore, warnings are provided for all types of misfires, the operator may mistakenly believe that the engine has some abnormality.